How to Monitor Growing Degree Days

Degree days are units of measurement reflecting just how hot or cold the weather has been. This is useful because insect, disease and weed pests develop more rapidly when temperatures are warmer. Growing Degree Days are those days necessary for these organisms to complete their growth and development.

Why measure Growing Degree Days?

By using degree days, you can time your spray according to the pest's temperature-driven rate of development, saving you money by applying chemicals when the pests are most susceptible. For example, your past experience tells you that on average, it takes 14 days for a specific insect pest's eggs to hatch. But in colder years, the eggs hatch after the 14th day, and in warmer years, eggs hatch sooner. Degree days express these temperature differences with greater precision.

How do you calculate Growing Degree Days?

There are various methods for calculating degree days, such as average, triangulation and sine. One simple method involves using the "lower developmental threshold" or "base temperature" and the "maximum threshold" for a specific insect pest or disease organism. This is called the Daily Averaging Growing Degree Day Method. The base temperature and maximum threshold information can be obtained from your local Cooperative Extension Service, while the minimum and maximum temperatures are obtained from local weather instruments.

Base temperature = the temperature below which the pest will not develop.Maximum threshold = the temperature above which the pest will not develop.Minimum temperature = the lowest temperature for the 24-hour period.Maximum temperature = the highest temperature for the 24-hour period.

For example, let's calculate the number of degree days accumulated on one day for an insect that has a base temperature of 55°F and a maximum threshold of 90°F. A min/max thermometer will show you that the maximum (high) temperature on that day was 86°F and the minimum (low) temperature was 62°F.

To calculate degree days for that day, you would add 86 + 62 = 148, then divide 148 by 2 = 74, then subtract 55 from 74 = 19. You would have 19 Growing Degree Days for that day.

When do you start?

Your local county Extension agent or land grant university has start dates and developmental totals for many pests. The start date would be the date on which to start your calculations. Calculate the Growing Degree Days for each day after the start date and keep a running total. When you reach the developmental total, you have reached the estimated emergence date for that pest.

What tools do you need for gathering temperature data?

A min/max thermometer, an on-site weather station and software, regional or site-specific weather reports, or a degree day recorder can be used to calculate and accumulate degree days. You can also get current Growing Degree Days information from some Cooperative Extension web sites.